How do they do it? Part 1

Guest post by KVR Instructor, Cathy Chybowski

As the days become shorter and the temperature drops, animals have three options in order to survive: migrate, hibernate or remain active. These strategies along with essential adaptations help animals to survive the cold season.   

 photo credit: freepik.com

Migration: An Easy Out?

Migration can be quite risky. The energy cost of migration is high and there can be many obstacles along the route. Most animals spend time in a feeding frenzy prior to migrating. This added fat will fuel their movement to an area with sufficient food. For hummingbirds, warblers, flycatchers and other birds that lose their food source with the onset of winter, migration is a necessary risk. Large bodies of water, a changing food supply, predators or hunting can make the trip challenging.

Did you ever wonder why it is mostly birds that migrate? Why so few mammals? According to Peter Marchand in his book, Life in the Cold, a mammal would expend ten times more energy moving a given distance by running than would a bird of equal weight flying that same distance. Bats and caribou are exceptions. For more information on bats and insects that migrate see a previous post I shared in August.

Hibernation? Dormancy? What is the difference?

Dormancy is one of the three strategies animals evolved for winter survival. It is a period of inactivity in which an animal’s bodily functions are slowed down. By definition, hibernation is an extreme form of dormancy. Woodchucks, bats and jumping mice are among the few mammals in our state to hibernate.  

 photo credit: WI DNR

As an herbivore, the woodchuck’s life is centered on the availability of plants. Feeding heavily during late summer into fall, it accumulates thick rolls of body fat and then retires to its burrow usually by the end of October. While hibernating, its heart rate drops from 75 beats to 4 beats per minute, its body temperature drops from 90 to 38 degrees, and it breathes once every 5 to 6 minutes. If the woodchuck can maintain this state of low metabolism for several months without freezing to death, it will usually emerge from its burrow during the first week of March (not like Punxsutawney Phil on Feb. 2). 

There are degrees of dormancy, from the true hibernation of the woodchuck at one extreme, to the deep sleep of bears and chipmunks, and to the short term sleep of skunks, opossums and raccoons. In all forms of dormancy, the metabolism slows down in order to conserve energy at a time when food is scarce.

   photo credit: bearstudy.org

Research shows that the black bear does not hibernate; its body does not undergo the drastic changes of a true hibernator. In order to prepare for its deep winter sleep, bears spend the fall in a feeding frenzy putting on several inches of body fat and growing a thicker fur. While in the den, the sow gives birth to 2-3 cubs which do not hibernate either. During this state of dormancy, the bear does not eat or drink, it does not produce waste and it does not lose bone or muscle mass despite the lack of exercise. How do they do it?  These unique adaptations are of interest to medical science. There is more to learn about black bears—how is it that they can survive a 5-month rest and fast and emerge from their den with a 99% survival rate?  Apparently there are few people willing to crawl headfirst into an active bear den in the winter to see just how cozy everything is in there!

    photo credit: WI DNR

Chipmunks spend much of the fall collecting seeds and nuts for storage in their underground burrow where they spend most of the winter in dormancy. The stored food gives them the option of waking and being active for a time; I was surprised to see a chipmunk out and about during a January thaw. No true hibernator could do this.

  photo credit: WI DNR

Unlike mammals, reptiles and amphibians are cold-blooded. Their options for winter survival are few. Since their body temperature drops with the outside temperature, they will freeze to death if they do not hibernate or go dormant in some form. Most frogs and turtles hibernate in the mud at the bottom of the pond or lake. Wood frogs are an exception, burrowing under damp leaves and into the soft soil of the forest floor. The wood frog produces an antifreeze which prevents its cells from freezing. Much of the body fluids outside the cells will freeze as will the frog’s eyes and brain. Its heart and lungs will stop. This “frogsicle” looks lifeless, but with the arrival of spring, it thaws quickly and is the first frog to emerge from hibernation. It is no wonder that the first frog we hear calling in the early spring is the wood frog. When the ice recedes and the water warms to 46 degrees, these frogs begin to call for a mate.

Snakes, cold-blooded reptiles, generally hibernate in dens or burrows of other animals, sometimes in groups of mixed species.  These are called balls of snakes and it is an amazing thing to see in the spring as these reptiles emerge from their hibernaculum. I witnessed dozens of garter snakes emerging this way one time in southeastern Wisconsin. I happened to be in the right place at the right time.

Stay tuned for Part 2, Animals Toughing It Out, in the weeks to come!